Kansas Sen. Pat Roberts survived a competitive Republican primary, but it looks like his toughest race is still to come. Democrat Chad Taylor dropped out of the race Wednesday, leaving independent Greg Orman as the senator’s main challenger and completely changing the math of the race.

A spectacular confluence of events has built the credible scenario that a Republican could lose a Senate race in Kansas. Roberts is a longtime incumbent who doesn’t live in or regularly return to his home state. He faces a credible and well-funded independent candidate who is striking all the right tones in his message and doesn’t have a legislative record to be picked apart. And GOP Gov. Sam Brownback has fanned the flames of a longtime civil war in the state that is rallying some Republicans against establishment figures within their own party.

For a little bit of a review, the Republican primary was the senator’s first real race in decades. Roberts needed outside help to ramp up his campaign operation to get to something even close to a 21st century effort. And even though physician Milton Wolf ended up being a flawed challenger and he failed to rally the biggest, anti-establishment outside groups to his cause, Roberts still only won, 48 percent to 41 percent, in the Aug. 5 primary .

But what might have been more stunning than the result was what Roberts’ longtime campaign manager Leroy Towns told The Wichita Eagle after the race was over. “He went back home for two days or three to rest. I think he’s going to come back here the first of next week,” said Towns, referencing Roberts’ home in Virginia. Towns’ comments seemed tone-deaf considering Roberts was dogged by residency questions throughout the race up to that point, and the general election was not completely certain with the threat of a well-funded independent candidate. “[H]e does intend to spend every moment between now and the election in Kansas, I think, that he can,” Towns also told The Eagle. But, according to Republicans familiar with the race, that just hasn’t happened. Unlike Mississippi Sen. Thad Cochran, who won an extremely competitive GOP primary and hasn’t stopped doing local events and making phone calls to supporters, Roberts has not been actively campaigning for about a month now.

The lack of a strong campaign infrastructure is one of the fundamental reasons why Roberts is in severe danger. He can’t count on the the traditionally red hue of Kansas in federal races to bail him out. Senate Minority Leader Mitch McConnell is a long-time incumbent with deep negatives, but the sheer past performance of Kentucky would not be enough to pull him to victory in this environment. He has been running one of the most aggressive campaigns in the country for years. The same cannot be said for Roberts.

Roberts is also vulnerable because he faces a very unique challenger. Orman, a businessman, ran for Senate as a Democrat in 2008 but dropped out during the primary. And he has contributed to Democrats in the past , including President Barack Obama. But Orman also doesn’t have an extensive legislative record that can be used against him, and he hasn’t been attacked for the last year, like McConnell’s challenger, Alison Lundergan Grimes.

Now that Orman is officially Roberts’ main challenger, Republicans will certainly do a deep dive into his record. And it may be a couple weeks before we find out whether there is a piece of opposition research that will doom Orman’s campaign.

For now, it looks like Orman is doing everything right. He’s running ads that cover nearly the entire state that strike a moderate tone. “I’m a businessman who solves problems every day,” he said in a recent spot, for example. And Orman unveiled endorsements from more than 70 former Republican lawmakers on Wednesdsay.

www.youtube.com/watch?v=lFYb0_Zidw4

The winning coalition for Orman includes consolidating Democrats (which is much easier without Taylor in the race), winning a majority of independents (who might be turned off by Roberts’ longevity and residency issues), and getting a chunk of disaffected Republicans who are fed up with status quo Republican officials in the state.

With Taylor in the race, Roberts may have only needed somewhere between 36 percent and 40 percent to win, because Taylor and Orman would have divided the anti-Roberts vote. With Taylor out of the race, Roberts will likely need closer to 48 percent of the vote, considering Libertarian Randall Batson is still in the race and could receive between 4 and 5 percent.

As of July , Republicans made up 44 percent of registered voters compared to 31 percent independents, 24 percent Democrats and less than 1 percent Libertarians.

Reaching the high 40s in the general election may not seem like a stretch for a Republican incumbent in Kansas, but it sure feels that way for Roberts considering the type of campaign he has run thus far.

So what happens if Orman wins?

Orman ran as a Democrat previously and he has a pair of Democratic consultants for this race (although the National Republican Senatorial Committee hasn’t looked too kindly on any Republican consultants challenging incumbents this cycle, so it may have been tough to find good GOP help), but the candidate has not said who he would caucus with next year. Orman assuredly won’t declare that before Election Day, because it would likely sink his own candidacy.

But Democrats appear to be willing to take a chance on an Orman victory (because he might caucus with Democrats), particularly compared to a Roberts victory (who will never caucus with Democrats).

If national Democrats run positive ads on Orman’s behalf, they would risk tainting him with independents and disaffected Republicans. But Democrats could run anti-Roberts ads with less of a connection to Orman but still helpful to the cause.

Roberts’ vulnerability is not a complete surprise. We recently moved the race out of Safe Republican to Republican Favored. But with the latest events and the current state of play in Kansas, we are changing The Rothenberg Political Report /Roll Call rating of the race from Republican Favored to Tossup/Tilts Republican . This makes Pat Roberts the most vulnerable Republican senator in the country.

The Republican establishment’s effort to pull Cochran across the line in the Mississippi Republican primary might be one of the most impressive feats of the cycle. But getting Roberts ready for prime time and through this general election could be an even bigger challenge.

With the potential for an Orman victory in Kansas, a likely December runoff in Louisiana, and a potential January runoff in Georgia, the make-up of the Senate next year could be in limbo for weeks after November.